When hypo-peritectic steel containing 0.08 to 0.18 mass % of C is continuous-cast, a solidified shell that is formed by solidification of molten steel in a mold tends to be unequal in thickness, which causes longitudinal cracks to easily form on a surface of a slab.
It is effective to mildly cooling the solidified shell (hereinafter, may be referred to as “mild cooling”) in order to make the solidified shell in the mold equal in thickness. It is relatively easy to use mold flux for mild cooling.
The mold flux is supplied on molten steel in the mold, and melts with heat supplied from the molten steel. The mold flux in a melting state flows along the mold, to come into a gap between the mold and the solidified shell, and to form a mold flux film (hereinafter may be referred to as “film”). Just after the casting starts, this film is cooled by the mold, to solidify like glass. Crystals are educed from the glass as time passes. When crystallization of this film is promoted, the roughness of the surface of the film in the mold side increases, which causes the thermal resistance at the interface between the mold and the film (hereinafter may be referred to as “interfacial thermal resistance”) to increase. Radiative heat transfer in the film is also suppressed. These effects allows the molten steel and the solidified shell touching the film to be mildly cooled down.
It is cuspidine (Ca4Si2O7F2) that is common composition of crystals educed from the film.
The following methods have been worked out as means for promoting crystallization of films:
A method for controlling fluid physical properties of mold flux, specifically, a method for raising the solidification point is effective for promoting crystallization of the film. About this method, Patent Literature 1 describes that crystallinity of a film is improved by raising the solidification point of mold flux to the range of 1150 to 1250° C. However, it is said there is a problem that if the solidification point of the mold flux is raised to 1250° C. or over, the lubricity is disturbed and breakout cannot be prevented.
A method for controlling components in mold flux, specifically, a method for increasing the ratio of the contents of CaO to SiO2 (hereinafter may be referred to as “basicity”) is also effective for promoting crystallization of the film. A method for reducing the MgO content in mold flux is also effective for promoting crystallization of the film. Concerning these methods, Patent Literature 2 discloses it is effective for crystallization of a film that in mold flux, the basicity is specified by 1.2 to 1.6 and the MgO content is specified by no more than 1.5 mass %. However, the highest temperature where the mold flux forms crystals disclosed in Patent Literature 2 is about 1150° C. at most, and only an effect of mild cooling corresponding to this is obtained. That is, the effect of mild cooling is insufficient.
On the other hand, Patent Literature 3 discloses a method for suppressing radiative heat transfer in a film by adding an iron oxide or a transition metal oxide to mold flux.
However, CaO, SiO2, and CaF2 in the mold flux are diluted by the addition of any of these oxides. Specifically, in Patent Literature 3, no less than 10 mass % of an iron oxide or a transition metal oxide in total has to be added as shown in its Examples in order to obtain a sufficient effect of suppressing radiative heat transfer. In this case, cuspidine is difficult to be educed when the composition has about 1.0 of the basicity shown in Examples of Patent Literature 3, and the solidification point of the mold flux drops. The solidification point shown in Examples of Patent Literature 3 is about 1050° C., which is lower than that in Patent Literature 1 by no less than 100° C. considering that the solidification point of the mold flux for hypo-peritectic steel suggested in Patent Literature 1 is about 1150 to 1250° C. As a result, crystallization of the film is blocked. Thus, with the art of Patent Literature 3, an effect of mild cooling obtained from increase of interfacial thermal resistance and the like according to the crystallization is marred.
Patent Literature 4 discloses a range of the composition of mold flux of the quaternary system of CaO—SiO2—CaF2—NaF where cuspidine is easily educed. The range of the composition is substantially same as a primary crystallization field of cuspidine as published in Non-Patent Literature 1 thereafter. According to the mold flux of Patent Literature 4 as described above, longitudinal cracks do not form on a surface of a slab when hypo-peritectic steel is rapidly cast, which makes it possible to obtain the slab which has a good surface quality.
Patent Literature 5 discloses a method for adding a transition metal oxide to the basic composition prepared within the range of Patent Literature 4, to drop the solidification point without marring an effect of mild cooling. Patent Literature 5 is aimed at the problem that when the Mn content in molten steel is high, crystallization of cuspidine is blocked because the MnO content in the film increases due to oxidation reaction of Mn, and thus the effect of mild cooling cannot be sufficiently obtained. For this problem, a necessary content of MnO is contained in advance, to suppress oxidation reaction of Mn, and then the solidification point is raised to a desired level. Whereby, it is possible to prevent longitudinal cracks on high-strength steel of the high Mn content from forming, according to Patent Literature 5.